Copernicus On The Science Of GRAVITY!!

I’ve already written a review of GRAVITY as a film in its own right. In short, I loved it, and I think it will go down as a landmark film in cinema history. But as promised in that review, now I want to go deeper into what GRAVITY got right and wrong about the reality of the space program. While my initial review was spoiler-free, be warned -- this one is not and you should see the film before reading it

Regular readers of the site know that I’m not just a movie geek, I’m also an astrophysicist. And from time to time I write articles about the science behind certain movies. The purpose isn’t to nitpick, but to broaden our whole geeking out experience, and to use the movie to talk about science. My general principle is that it doesn’t bother me when filmmakers have to bend reality to tell a great story. But I can’t stand it when they get the details wrong out of laziness, or produce films that make no sense.

Aside from astrophysics knowledge, what makes me qualified to talk about GRAVITY? I’ve spent a lifetime of being a space geek, and growing up in Florida, I would travel to see Shuttle launches, read everything I could get my hands on, and even pore over every detail in the Space Shuttle Operator’s Manual from the library. That was all great preparation for an amazing gig I was lucky to get as the cohost of a TV show (with astronaut Mike Massimino and others), the third season of National Geographic’s KNOWN UNIVERSE (now on DVD, Amazon) that was in equal parts about astronomy and space exploration. In filming the show, I got to go to behind the scenes at several NASA facilities, watch astronauts train, and play with their toys. I talked to many astronauts, but just as importantly got to interview the scientists and engineers who design the spacecraft, space suits, tools, and simulators that keep the astronauts safe. I put on a space suit, ate the space food, and got in the simulators. It was great fun, and though I didn’t know it at the time, excellent background for understanding all kinds of things going on in GRAVITY from an insider’s perspective.

In this article I’ll mostly go in chronological order through the film, taking a look at what the filmmakers got right and wrong, but mostly using it as an excuse to talk about space exploration and physics.

Blue Marble

After the title card, the first shot we see is of the Earth from space. It looks amazing, especially in IMAX 3D. You can see how massive and yet fragile it is. We all depend on that thin blue layer of atmosphere to survive. As the film progresses, we see majestic shots of mountain ranges, hurricanes, and even night shots of city lights and cascades of aurorae. These are based on real shots taken by astronauts over a half-century of space exploration. Just check out some of this stunning time-lapse video:

Continuing in the establish shot, we see George Clooney’s character, Matt Kowalski, is zipping around in some kind of experimental jet pack. This is based on a real thing! In fact, there have been a whole series of space jet packs dating back to a prototype system that was sent up in the Gemini days, but never used. In the 1980s, shuttle astronauts did use a system that looked and acted almost entirely like Clooney’s system, the Manned Maneuvering Unit, or MMU. It shoot out jets of compressed gas which let the astronauts maneuver in space, untethered. Astronauts used them to capture satellites, and they were expected to be used in space construction, like building the ISS. But after a few missions, their use was discontinued. They worked fine, but if there was a malfunction the astronaut could shoot off into space, never to be seen again.

In GRAVITY, the astronauts’ motion is sped up from what it would be in real life. Astronauts take slow, precise, deliberate actions in space, because speed is your enemy, both in terms of reaction time, energy of impact, fuel consumption, space suit constrictions, and our inherent unfamiliarity with microgravity. But hey, that’s a concession I’m willing to make -- I’d rather have a movie that moves along than a perfectly accurate, plodding one. I’ve watched enough hours of NASA TV to know that real spacewalks aren’t exactly your ideal suspense thrillers.

Today, every astronaut on a spacewalk remains tethered or attached to an arm, but as a backup they also have a simplified version of the MMU called SAFER for Simplified Aid for EVA Rescue. This is for precisely the kinds of situations Sandra Bullock’s character, Ryan Stone, found herself in. A real-life astronaut would have been able to use the thrusters on the SAFER module to right herself and return to the shuttle after being separated. But there isn’t much fuel, so she couldn’t go much farther than that. I can understand why this kind of thing was left out of GRAVITY -- it would just complicate the narrative. Clearly in the universe of GRAVITY, they are so confident in the MMU that George Clooney uses in its ability to rescue stranded astronauts, that they don’t require all astronauts to wear the comparatively weak SAFER modules.

Arm wrestling

GRAVITY mostly gets the details of the physics right. I love the scene where Sandra Bullock’s character, Ryan Stone, is spinning end-over end, unable to right herself, and we see it from her perspective. That’s what it would be like! In microgravity you lose the sense of up or down that the Earth’s gravitational field gives you. So from her perspective, the Earth is just flying through her field of vision in loops.

But they did get some very big things about the physics wrong. In the initial accident, she’s goes flying while attached to the Canadarm. Matt tells her to detach from it or it will carry her too far away. That is exactly the wrong thing to do. The rotating astronaut + arm has a certain amount of angular momentum. Without getting into the equations, more spread out things rotate more slowly, and more compact things rotate faster. That’s why when ice skaters bring in their arms they speed up. By detaching, Ryan switches from a long lever arm rotating around the common center of mass of her + Canadarm into a more compact configuration. And a lot of that rotational momentum would be converted into linear momentum. Plus, her mass gets lowered by losing the arm. The result is that she’d be shot out at a much higher speed. They still could have had her panic and do this in the film, but they should have captured the physics a bit better, and they shouldn’t have had the more experience astronaut (who would have known this) to tell her to do something stupid like that.

This is parodied in this poster, which I think is by astrophysicist Katie Mack:

The next big physics gaffe is when George Clooney’s character has to let go to let Sandra Bullock’s character live. The idea is that their joint momentum is too much for the parachute lines she’s got her foot in. The formula for (linear) momentum is just mass times velocity, so it is true that the two of them together have roughly twice the momentum that one of them would have. But as shown in the film, she seems to have already pulled the parachute lines taut and stopped their joint forward velocity. There is really no reason to let go at that point. But this is a case where we can’t get too nitpicky. Clearly, you can imagine what they were going for, even if it wasn’t perfectly achieved in the visuals. And the characters have to have some time to say their goodbyes. So we have to allow a little dramatic license.

A bad case of Kessler Syndrome

The inciting incident in GRAVITY is that a cascade of space debris takes out the Space Shuttle. This is based on a real phenomenon, or at least a plausible scenario, called the Kessler syndrome, identified by NASA scientist Donald Kessler in 1978. The idea is that a few large collisions between objects in space could result in an ever-growing cloud of space debris that could render near-Earth orbit unpassable.

It is true that a big space collision can produce millions of pieces of space debris. Those then set off other collisions, and they become a cascade. And because of the large velocities of space debris, in excess of 20,000 miles per hour, even a tiny speck of debris could kill an unprotected astronaut. That’s because the formula for kinetic energy is E=1/2 mv2, where m is mass and v is velocity. A bb moving at those speeds would tear through the Space Shuttle. I saw this firsthand at NASA White Sands, where they have a 40 foot long “hypervelocity gun” that they use to shoot bb-sized projectiles at targets meant to represent the spacecraft. That’s where they test the shielding for the ISS. A little bb can put what looks like a large caliber bullet hole through multiple layers of metal. In the picture below I’m inspecting such a hole after we pulled it out of the test chamber.

They put many layers of metal like this in front of the most sensitive parts of the ISS to absorb the kinetic energy of any space debris. But that still wouldn’t save it from larger debris, much less a full-on avalanche like we saw in GRAVITY.

The Kessler Syndrome is real in that we’ve seen the amount of space debris grow over time due to space collisions. However, it isn’t as large as Kessler predicted in the 70s, partially because we’ve been lucky and there haven’t been many big collisions, but also because the Soviet Union collapsed, and their launched rate dropped as a result. But big events still are a worry as a precipitating factor. In 2007 the Chinese launched an anti-satellite weapon at one of their own satellites. The US has done the same. In the film the Russians are the bad guys.

One thing that isn’t accurate about the cascade we see in the film is that it would really take years to spread around the Earth and grow, not the minutes we see in the movie. Again, this would make for a pretty boring film, so I’m ok with the dramatic license.

It wasn’t just a coincidence that our heroes kept getting pounded by debris. Matt tells Ryan to set her watch to 90 minutes, since that is when the debris will return. He says something like, “Factoring in that it is going at 50,000 miles per hour...” This part is nonsense. Orbital debris moves at more like 20,000 miles per hour, and the time it takes to circle the Earth is fixed by the semimajor axis of the orbit (you can calculate this using Newton’s form of Kepler’s third law!). Ninety minutes is a typical low Earth orbit -- the Hubble Space Telescope, the Space Shuttle, and the ISS all orbit the Earth with about that period. (If you see HST or the ISS in the night sky, you can really tell that they are zooming along.)

The fact that the debris returns every 90 minutes means that it isn’t moving along with the astronauts, it is in an orbit somewhat perpendicular to the Space Shuttle and space station orbits. It must be a big long cloud to increase the probability of hitting our heroes multiple times, because if it is small, the chance that you’ll hit twice in two orbits is small. But it can’t be a full ring of debris encircling the globe, or else our heroes would have hit it every 45 minutes.

Another cool thing is that since the astronauts get hit with debris 3 times, we can tell that this film happens over 3+ hours, not just the 90 minutes we see on screen. This is in some ways an acknowledgement that some scenes were sped up or edited from what we would have seen in “real life.”

Ground Control to Major Tom

In the movie, the debris cloud apparently knocks out communications satellites, so that contact is lost with mission control. It is true that since the 1980s, NASA has used a network (there are currently seven) of Tracking and Data Relay Satellites (TDRS) to communicate with astronauts. These allow more uninterrupted communication than the old series of ground stations used in the Apollo era. Despite this redundancy, it is possible to lose contact with them as happened to the ISS earlier this year due to a computer glitch. However, when that happens, NASA can still use the old ground station method -- when the ISS was out of contact with TDRS, they had sporadic communication with the astronauts as they passed over Russia. For GRAVITY, ignoring ground stations doesn’t bother me so much -- maybe in the film’s chronology they have been eliminated due to budget cuts.

What really bugs me is the apparent destruction of the TDRS satellites. They are in geosynchronous orbits. This means that they stay over roughly the same spot on Earth, since they orbit with the same period as the Earth’s rotation. To do that, they have to be *way* up there, about 22,000 miles above the Earth’s surface, or almost a tenth the distance to the Moon. This is compared to to low-earth orbit, where our astronauts are. HST is about 350 miles above Earth, while the ISS is only 230 miles up. So TDRS satellites are roughly 100 times farther away than the Hubble Space Telescope or the ISS. If the debris is moving at 50,000 miles per hour, as they state in the film, then it would take at least 26 minutes after the strike on the Shuttle to start taking out TDRS satellites. But it gets worse than that. The same debris from low Earth orbit would be spread out over a much larger volume at geosynchronous orbit, so would have a very low probability of taking out TDRS satellites. And this is further compounded by the fact that this is an orbiting, somewhat isolated cloud of debris -- it seemingly hasn’t affected all of low Earth orbit as the events of the film unfold. And it is spread out to geosynchronous orbit, it too would become geosynchronous, so this Kessler effect would lessen greatly. The Kessler Syndrome really only applies to low Earth orbit.

To be fair, I’m assuming that astronaut communication in the film works the same as it does in our universe. But they never say it does.

Orbituary

The largest suspension of disbelief necessary to enjoy GRAVITY is to embrace its total disregard for the amount of fuel it takes to change orbits. In the film, our heroes go from an orbit where they are servicing the Hubble Space Telescope (HST) over to the International Space Station (ISS) with only an experimental jet pack. In reality, you can’t do this -- even the Space Shuttle can’t get to the ISS after servicing HST. This isn’t just a pedantic point -- this had huge implications for space policy and all of astronomy, my research included.

After the Columbia disaster, a policy was adopted where all Shuttle missions had to be able to reach the ISS in case of mishaps. So, in 2004 NASA administrator Sean O’Keefe made the tough call to kill the final servicing mission to HST on the grounds that the Shuttle could not make it to the ISS. Scientists, astronauts, Congress, and the public rebelled, and as a result, the servicing mission, arguably the most important thing the Shuttles did, was reinstated. In 2009 a team including my friend Mike Massimino, risked their lives to give HST new life. I directly benefitted, since I used the Space Telescope Imaging Spectrograph to observe the closest supernova in a generation soon after. That’s one reason I have such admiration for Mike -- he was a great cohost, but more than that, I owe some fraction of my career to his bravery and skill. And that’s one reason I consider the leap in GRAVITY from HST to the ISS no small thing.

But that holds for the orbital transfer of the massive Space Shuttle. Could two much lighter astronauts and a jet pack get to the ISS from HST’s orbit? We can calculate this! Contrary to popular belief, rocket science isn’t that hard. We use the rocket equation, which tells you the mass of fuel you need to achieve a change in velocity of an object of a certain mass.

The mass of fuel (mf) we need is:

mf = ma x (exp(dv/ve)-1)

Where ma is the mass of our astronauts and their gear, dv is delta-v, how much you need to change velocities by when switching orbits, ve is the velocity of the exhaust (how fast stuff shoots out of our jet pack), and exp is the exponential function.

First we need to know the mass of our two astronauts in space suits, plus jet pack. Based on Wikipedia and what I was told at the NASA space suit lab when I visited, that is easily estimated -- it is about 550 kg.

Now for delta-v: how much velocity do we have to add or subtract from our astronauts to get them to change orbits? The ISS orbit is different from HST’s in two ways: it is at a different inclination (tilt with respect to the equator), and it is at a different altitude. As detailed in this pdf, it turns out the first term dominates, so we basically need to change the velocity of our astronauts by about 3.1 km/s to have them transfer orbits. That’s about 10 times faster than a speeding bullet, so we are talking about a Superman-level feat here.

We really need a lot of thrust and a lot of fuel. We need an exhaust velocity comparable to the change in velocity we need to transfer orbits we mentioned above: 3.1 km/s. When you want a lot of motion in space you create a chemical reaction that releases energy. For example, in the Space Shuttle Main Engines (SSMEs), liquid oxygen and liquid hydrogen are mixed to make fire and water when blasting off from Earth. The SSMEs have exhaust velocities of 4.4 km/s, which is great!

But the problem is that they are massive, complicated, hot, and require a giant external tank of fuel. Not so good for a jet pack. For quick motions on a smaller scale, like in a jet pack, compressed air is shot out at high pressure. That’s what happens in GRAVITY -- they say so in the film. It isn’t a chemical reaction -- it is just like holding your finger on a water hose to shoot the water out at higher pressure.

Unfortunately, compressed air jets fall short of what we need by a factor of about 100 to 1000. And it is even worse than that: the delta-v term is inside the exponential in the equation. Plugging this all in, using jet-pack jets, you’d need more fuel than exists in the universe. In short, it can’t be done.

But what if George Cloony just trapped a space shuttle engine to his back? Even then, he’d need a big tank with about 600 kg (1300 lbs) of fuel.

You may say: so what? They bent the rules. Big deal. But to astronomers, physicists, and astronauts who know these things, it is as crazy to us as two characters getting into a car and driving to the moon. On a single tank of gas. Even still, I’m willing to put up with it, because without it there is no movie, and GRAVITY is a great movie. It is a suspension of disbelief, not much different with accepting that Superman can fly. Having said that, there is a bit of an out, which I’ll discuss later in the section on the Chinese space station.

Newton -- It’s the Law

Newton’s third law of motion says that for every action there is an equal and opposite reaction. That’s the one that makes rocketry possible. Shoot some gas one direction, and it sends you in the other direction. This happens on Earth too -- it is why shotguns have kick, for example. But mostly we don’t notice it because we are bound by gravity to the Earth, and friction keeps us there. If you throw a ball, you don’t fly backwards, because of the friction of your feet against the ground. But try it on ice skates.

Newton’s third law has interesting implications when you are in microgravity though. When astronaut Gene Cernan went on one of the first spacewalks, almost everything went wrong. He’d try to turn a latch handle one direction, and find that he would go flying in the other. He explains it in this video:

Interestingly enough, Cernan was also testing out a “zip gun” that shoots out compressed gas as a way to get around in space. This isn’t too different from the fire extinguisher scene in GRAVITY.

In the movie Sandra Bullock rotates handles with wild abandon, and doesn’t spin the other direction. This is partly because they filmed these scenes on Earth. But they do acknowledge Newton’s Laws by having her always holding on to something while operating a handle. You’d have to do that to get leverage anyway. Similarly her hair doesn’t fly up in the air, because she was really filming on Earth. But at least they gave her a short haircut to make this less obvious.

But those are ridiculous nitpicks. My overwhelming feeling is that they did an amazing job on the weightless scenes. I was continuously astounded. How did they do that? On Apollo 13 they filmed a few scenes in NASA’s training aircraft that flies in parabolas to simulate weightlessness -- the “vomit comet.” But they didn’t do that here -- the sets have to be tiny and you only get a few minutes of weightlessness in one go -- not Cuarón’s style. Here they did it with a combination of computer graphics, 12-point wire harnesses, and inventing new technology: a giant rig with hundreds of LEDs that actors had to film their scenes in.

For more on how it was done, see this excellent article by Alan Boyle over at NBC News. They also talk about the process briefly at this Q&A after the Toronto premiere, where, interestingly enough Canadian Astronaut Cmdr. Chris Hadfield shows up. If you haven’t seen him singing SPACE ODDITY from the ISS watch it now! It is the greatest music video ever made.

At least she’s not an oil driller, but...

Actually the biggest problem I had with Gravity was the way they treated astronauts. Sandra Bullock’s character is said to have had only six months of training! In reality, NASA astronauts get many years of training before each mission. I know they did it to make her seem more vulnerable, since she wouldn’t know what to do. But that was unnecessary. I know real astronauts who have been scared shitless in space.

I know the ostensible reason is that she has a “prototype” that she’s installing in the Hubble Space Telescope. This is also pure silliness. NASA designs space hardware many years in advance, and all the components have to be vetted for space. I’ve sat in on some of these committees designing space hardware and they are fascinating, but tedious. Materials are carefully selected for their properties of thermal expansion and outgassing. Every component has to be radiation hardened, which usually means using a decade or older computer processors. Even the kind of glue you can use matters. For example, instruments installed on HST servicing missions often were designed a decade before.

Making matters worse, Sandra Bullock’s character isn’t even an astrophysicist (as I thought after seeing the movie once) -- she works at a hospital. This really strains credulity. I know they say it is hospital technology that has been adapted for HST, and yes, somehow that is remotely possible. One of my colleagues has tried to adapt our supernova search algorithms to cancer screening, for example. But medical doctors (if that’s what she is), or even hospital equipment manufacturers, do not design telescope-worthy instruments. I’m sure they were going for some thematic link between looking inside humans and looking into the universe (we are all starstuff and all that), but it wasn’t worth the suspension of disbelief required for such a small point.

And it doesn’t matter that she built the prototype. Different people build and install the instruments. You get awesome instrument builders to make great instruments, and awesome astronauts to do the installation, who train for that for years. What’s more, Sandra Bullock’s character has some serious psychological issues. She’d never pass the psychiatric evaluation.

Another maddening detail is that in the film, George Clooney’s character doesn’t even know where Sandra Bullock’s character is from, or many details about her life. In reality, NASA selects crews for their compatibility in terms of personality, and has them spend lots of time together on Earth outside of training, even going on camping trips so that they better understand each other and their group dynamic. You have to know how your fellow astronauts are gong to react in any situation, and trust them completely because your life may depend on it. In fact, Mike said, when you get to the top hundred or so astronaut candidates, they are all outstanding, and a large part of the decision making process from then on involves personality and crew compatibility.

Great balls of fire

Fire burns differently in microgravity. It is pretty awesome. Instead of a teardrop-shaped flame that is blue at the bottom and orange at the top, like we get on Earth, in space you get a spherical, blue flame. That’s because in the absence of gravity the heated air doesn’t rise, to be replaced by fresh air. The flame looks feeble, but can in fact be hotter, and can reach temperatures where metal is melted. We know this because astronauts have done experiments with fire on the international space station.

In GRAVITY they get this right -- when she first passed by the tiny fire on the ISS, it was burning with a spherical flame. I geeked out. I think they might have colored the flame a bit more orange and had it brighter than it would otherwise have been, but that’s ok. First and foremost, it has to register as fire to the audience. They are showing the experts that they know the physics, but simultaneously showing the audience what they need to see. That’s perfect.

Astronauts do have experience with fires out of control in space too. In 1997 a fire nearly destroyed Mir. The Russian astronauts were so shaken that they basically said “fuck it” and had a party where they drank cognac in space. That led to this great photo of slightly lit astronauts. It is the only one you’ll ever see, because it was never meant to see the light of day.

Yes, Russians are officially are supplied with cognac, not vodka. Who’s to say that they don’t also sneak a little vodka on board too? Again, artistic license. It works better as vodka. Sadly, Americans are not allowed by NASA to drink alcohol in any form, as I learned on my trip to the NASA food science laboratory. Beer is especially forbidden, since the gas bubbles don’t work the same in microgravity and form some mass of gas in your stomach that will really screw you up.

Spherical flames also mean that fire extinguishers have to work slightly differently in space too. In a normal CO2 based fire extinguisher, the carbon dioxide will sink because it is lighter than air. But in microgravity it wont, so you just have to make sure you are really displacing the oxygen with CO2. Still, space fire extinguishers are based on CO2, which is integral to the plot of GRAVITY. Ryan bails out of her Soyuz capsule and uses a fire extinguisher to arrest her motion to the Chinese station. I’m not even going to calculate whether that was possible, because that scene is just too awesome.

I’ve even used a fire extinguisher as a form of locomotion on Earth. Every year in my introductory astronomy class I sit in a wagon and use a fire extinguisher to propel me across the lecture hall to illustrate Newton’s Third Law. You can achieve meters per second velocities, even on Earth!

A sharp dressed woman

The space suits in GRAVITY are pretty accurate. From the sleeve mirror to the Vader-like environmental controls on the front, to the snoopy cap, they got all the externals right. But the undergarments are all wrong. Going from the inside-out, US astronauts wear a diaper, blue pajamas designed to wick away sweat, and tight fitting white outer garment covered with tubes through which water circulates to keep you cool. That last bit is super-important -- I was about to overheat in my space suit until they turned on the water cooling system. Here’s me in the cooling suit, before I got in the space suit (that’s Mike Massimino on the left):

As far as GRAVITY goes, I have to defer to artistic license. Getting to see more of Sandra Bullock is an idea I’m 100% behind. And the more minimal clothing was easier to CG and help hide that it was filmed on Earth. Besides, it also works as a callback to ALIEN, makes her seem more vulnerable, and works better with that whole rebirth thing.

At the end of the movie she has to exit a waterlogged space suit in a hurry. I can tell you it doesn’t go that quickly, even with a team of experts in the best of conditions. At least that is true for US suits, though she was wearing a Russian suit. Going into and out of airlocks also can’t be done quickly. Here’s a checklist of the things you need to do to put on or take off a suit.

Heavenly Palace

After the Soyuz escape is a bust, our protagonist tries for the Chinese space station. Yes, there really is a Chinese space station, although it isn’t what you see in the movie. In 2011, a sort of test station was launched, Tiangong-1. It is a tiny thing -- not much bigger than a Soyuz spacecraft. By 2015 there are plans to launch a larger station, Tiangong-2, and by 2022 or so, the even larger Tiangong-3. I think it is something along the lines of Tiangong 3 we see in GRAVITY.

So now we have a timeline problem. The last HST servicing mission happened in 2009. The US Space Shuttle program ended in 2011. And the Chinese station shown in the film won’t be up for another decade or so. The answer is obvious: this isn’t “our” reality shown in the film, it is an alternate reality set a few years to a decade in the future. Further evidence comes from the name of the Space Shuttle: Explorer. It isn’t one of the six that were in the US fleet (Enterprise, Columbia, Challenger, Discovery, Atlantis, Endeavour). And each mission is given a number: STS-XXX where STS stands for Space Transportation System. The last shuttle flight was STS-135. But the mission in GRAVITY is STS-157. That’s 22 missions in the future. We launched 130 Shuttle missions (the first 5 test flights of the Enterprise didn’t go to space) over 30 years, or 4.3 missions per year. So it would take about 5 years to get up to STS-157, longer if there were a disaster.

Could that give us an out? Maybe in this reality the Hubble Space Telescope, the International Space Station, and the Chinese Space station are all in the same orbit. In fact, they kind of have to be, given the events that transpire in the film. Is this plausible?

If you launch near the Earth’s equator you get an extra boost from the Earth’s rotation. (Hence our launch site being in Florida, at +28 degrees latitude). But the Soviets did not have an equatorial launch site, so they put their cosmodrome about as far south as they could, at Baikonur, Kazakhstan. That is at +46 degrees latitude, meaning you can’t launch things into orbits from there at inclinations (an angle measuring how tilted the orbit is with respect to the equator) less than that. The ISS was always intended to be built with Russian cooperation, so it was launched into an orbit that the Russians could access, with an inclination of +52 degrees. So the ISS has to be in that orbit. And since the Tiangong-3 is yet to be launched, let’s just say the Chinese will put it into the same orbit. The current Tiangong-1, which will be deorbited and won’t be a part of Tiangong-3, is in an orbit with an inclination of +43 degrees. But what about HST?

Maybe in the GRAVITY universe, once it was decided that astronauts had to be able to make it to the ISS on any mission, maybe the decision was made to add a booster to HST and move it to the ISS orbit. This isn’t a crazy idea, as this pdf lays out in glorious detail. There are design reasons to have HST in the orbit it is in, but I’d guess these aren’t insurmountable. After all, there are many spy satellites in polar orbits, and HST is very similar in design to the KH-11 class of spy satellites.

Space hallucinations

When Ryan is stranded in the Soyuz and can’t get the descent engine to work, she decides to just kill herself by cutting the amount of oxygen in the capsule. This is not necessarily the smartest thing to do because she has no idea whether someone could be coming to rescue her from the Chinese station. But she has only an intermittent will to live anyway.

It isn’t clear exactly what she does, because just cutting off the oxygen supply would work very slowly (remember how long she was going on just the oxygen in her suit). But to do anything that would work quickly she’d have to turn down the pressure in the cabin, and that ought to require venting some of the capsule’s atmosphere to space. At any rate, she does so, and starts to hallucinate. I really like this scene, because it gives us both a fake-out, and an alternative ending to the movie.

We are meant to think she wakes back up, figures out a solution, and rescues herself. But another interpretation is that the rest of the movie is a hallucination, and she does die after she turns down the oxygen. In fact, this is the more physically plausible interpretation, since you are unlikely to wake back up after you pass out from a lack of oxygen, if the oxygen level does not increase.

I don’t think the filmmakers meant that to be their intended ending, but it is interesting that it is there. If you are a cold, hard realist who thinks the film has too many implausible scenes to be taken seriously, then you can still enjoy the film on its artistic merits and laugh at all those people who fooled themselves into thinking she lived.

But not me. This is a time when the human spirit triumphs over plausibility!

Chinese take-out

When our hero finally gets to the Chinese space station, it is losing altitude due to drag from the upper atmosphere. It is true that all low earth orbit satellites experience some amount of drag, even from the unimaginably tenuous upper regions of the Earth’s atmosphere, and have to be boosted from time to time. Every time astronauts service HST, they kick it up into a higher orbit using the Shuttle.

But the dragging-down process takes years. In GRAVITY, the Chinese station is losing altitude at a furious clip. Something must have happened to push it way down in the atmosphere. The implication seems to be that the debris cloud that has been orbiting the Earth has somehow done it. But this doesn’t make much sense. The momentum from being hit by the side, or from below, is going the wrong direction. And even if some debris hit the station from above, it wouldn’t just push it down, it would set it spinning too. Maybe the spinning was stopped by the drag. Or maybe some thruster system got damaged and malfunctioned, causing it to stick open. Still, it was pretty weak not to give any reason for the improbably fast loss of altitude of the station. But at least it made for very cool effects and a novel station destruction scenario.

The pilot has turned off the no smoking sign

In GRAVITY they say the Chinese spacecraft, Shenzhou, has the same landing protocol as the Russion Soyuz. Indeed it was designed on Soyuz, but made bigger and newer.

Ryan says she crashed the Soyuz simulator every time. It seems obvious NASA would not send up such an incompetent astronaut. But maybe if she was only going up to service HST, they wouldn’t require her to learn all the Soyuz protocols. After all, you are supposed to learn Russian, and that is a huge overhead. But since in this universe the HST and ISS are in the same orbit, it seems logical that they’d require that and just send up a well-trained astronaut. The movie would have been effectively unchanged if they’d have just left out that line about her crashing Soyuz (actually it would have made the movie better).

In the end, Dr. Stone has to make an unplanned entry into the atmosphere. The Shenzhou, like Soyuz, comes in 3 parts -- the orbital, descent, and service modules. They are designed to be detached before reentry, for all kinds of reasons, not the least of which is that the heat shield is sandwiched between two of the components. But Ryan starts coming in with all the parts attached, leading to the capsule starting to burn up and the cabin filling with smoke.

It seems like one of the most implausible things in the movie, but remarkably, it is based on a real event. In 1969, on Soyuz 5, the service module did not separate, because of the failure of an explosive bolt. The spacecraft started reentry facing the wrong way, with the unshielded escape hatch facing forward. Temperatures during reentry reach as high as the surface of the sun, and without a heat shield, you will die. Boris Volynov’s story is riveting. He knew what was happening -- seeing the spacecraft starting to melt, the cabin filling with smoke, feeling the extreme heat. He was sure he would die, but then the atmosphere burned through enough of the service module that it was torn off and the spacecraft righted itself. But that still wasn’t enough to guarantee survival.

His thrusters had run out of fuel, so they couldn’t stabilize the craft from spinning, resulting in tangled parachute lines. His soft-landing thrusters (yes those are real) may have also failed. He hit the ground so hard, he got flung out of his seat and knocked out his teeth. He also landed far off course in the Ural mountains. Temperatures were -40 degrees, and rescuers would be a long time coming, so he was in danger of freezing to death. He set off on foot toward a plume of smoke in the distance. When rescuers arrived, they found an empty capsule, but followed the trail of blood to find him in a peasant’s cabin.

And this has happened more than once! Soyuz TMA-11 also failed to separate properly, nearly burned through the hatch, and landed off-course. One sexist Russian official blamed it on the predominantly female crew instead of the failed explosive bolt.

In GRAVITY, Ryan is facing a wrong-way entry and her capsule starts to burn through and fill with smoke. She separates the Shenzhou components just in time through, and the descent module rights itself. In the two real cases mentioned above the radio antenna has also burned through, but maybe in GRAVITY the damage wasn’t as extensive. That allows us to have a more fitting ending too, where we know she’s going to be rescued, but we don’t have to see it.

One if by land, two if by sea

Unlike Apollo capsules, the Soyuz capsules are designed to come down over land. This can present problems if you land off course. Allegedly in 1965, some wolves were spotted heading towards a Soyuz capsule landing site. Since then, the Russians have been packing heat on space missions! Until 2006 the Soyuz capsule was equipped with quite possibly the coolest gun ever made, the TP-82. It was a combination pistol, shotgun, rifle, flare gun, machete, and shovel, and came with a utility belt of ammunition. It was housed in a canister between two of the seats. These days, due to a lack of ammunition for the TP-82, cosmonauts (and Americans flying Soyuz) just use a regular semiautomatic pistol.

GRAVITY solves this landing problem by having our hero land in water, but just on the edge of land. Soyuz and Shenzhou capsules can land in water, but only as a contingency. This happened in 1976 on Soyuz 23, when they splashed down in a partially frozen Lake Tengiz. Low on power, the cosmonauts could only afford to keep on a small light inside while they waited through the night, partially submerged, for a recovery that took nine hours.

This is a neat solution for GRAVITY, because American audiences are more used to a splashdown. And water is useful in all kinds of thematic ways. It represents the unconscious, purity, and rebirth. And it gave us another fit of drama as she struggled to get out of her suit. The camera focusing on a frog was a nice juxtaposition -- frogs are perfectly adapted to their environment, but we have to use our intellect to take the environment we’re adapted to with us.

A space capsule suddenly filling with water also has a precedent in the space program, as anyone who has seen THE RIGHT STUFF can attest. When Gus Grissom returned to Earth in Mercury-Redstone 4, his hatch blew too soon and his capsule sank.

In GRAVITY when the cabin was filling with smoke, why didn’t Ryan just put on her helmet and pressurize her suit? Or why didn’t she do it when the cabin filed with water? She’d have been able to breathe and float too. Well, even Gus Grissom almost drowned when he forgot to close his suit inlet valve in the water and found himself sinking.

Thematically, and from an action perspective, getting rid of the suit makes for a much better movie. At the end she’s cast off all of the trappings of space and is standing nearly naked on she shore. We are reminded of the title card. “Life in space is impossible.” But life here on Earth? We’re made for it.

Final thoughts

People often get confused about the purpose of these articles. There is no one-to-one correlation between getting every nitpicky detail right and a movie being good. What I want is for filmmakers to know the details about the world they’re exploring, and then chose to take some artistic license where they have to for the purpose of making a better film. GRAVITY excels at this. The technology is all based thoroughly on the real world of the space program. Many of the things that happen to the characters are based on real-world events, or in the case of the Kessler Syndrome, real-world concerns. The result is a staggering level of immersion that it feels like an IMAX documentary as much as it does an IMAX feature film. Sure they sped some things up, ignored some safety protocols, cheated a bit on the physics, and screwed around with some orbital mechanics. But they deliberately did it to improve the film, and gave us a little wink at the same time, without going into long pedantic scenes about it. The result is something tight, lean, and dramatic: cinema stripped down to its core, and reinvented.